Narita is seriously short of capacity. KAL provided 48 weekly flights at 1988, 35 to Seoul, 6 to Pusan and 7 to LAX, but now it has only 41 flights. Of course AAR is founded since then, but AAR only operates 6 weekly flights. That says Narita can't attract more passengers, and even now it has just 50s weekly domestic flights. You cannot say Narita as a top airport.
Is one of those Asian airlines fly to Narita?: Air Macau, Vladivostok Air, Uzbekistan Airways, MIAT and China Northwest, all of those are already in Seoul. ANA moves long-hauls from Kansai to Narita, and the shortage of capacity grows.
So, has Pudong taken over even Seoul? Chinese cargo flow is greater in Seoul than Pudong, and Peking (or Shenyang, Qingdao, Dalian, and so.)-Seoul-US transfer is far more powerful than that of Shanghai. Shanghai's greatest weakness is absence of major airline. One of KAL and CPA can beat all of Air China, China Eastern and China Southern. And the No.1 in Shanghai, MU, is still mostly run in Hongqiao. Don't forget world 2nd international cargo airline and other 46 airliners in new airport.
I think Mega-Hub in Asia is mere dream of govts, but if it can be, it must be IIA.